1. Field of the Invention
The present invention relates to an optical disc drive for optically reading and/or writing data from/on various types of data storage media such as an optical disc (which will be referred to herein as an “optical disc” collectively), and more particularly relates to an optical disc drive that can make a tilt correction on a light beam.
2. Description of the Related Art
In an optical disc drive for reading and/or writing data from/on an optical disc by means of a light beam, if the data storage layer of the optical disc has tilted with respect to an objective lens, then the optical axis of the light beam will define some tilt angle with respect to a normal to the data storage layer. That is to say, in that situation, the incoming light beam is no longer perpendicular to the data storage layer of the optical disc. Once such a relative tilt (which will be simply referred to herein as a “tilt”) has been formed, the light beam impinging on the data storage layer comes to have some optical aberration. Such a tilt-induced aberration increases proportionally to the third power of the numerical aperture of the objective lens. For that reason, in order to read and/or write data from/on an optical disc just as intended by using a latest optical disc drive in which the objective lens has an increased numerical aperture to write data on the optical disc more densely, it becomes more and more important to correct the tilt of the optical axis of the light beam for use in the read and write operations.
An optical disc drive normally includes an optical head, which can move in the radial direction of a given optical disc (i.e., in the tracking direction) and which includes a correcting mechanism with the optical axis tilt correcting capabilities described above. More specifically, the optical head usually includes a movable body, which supports an objective lens thereon and which is provided with multiple coils. In this optical head, when a drive current is supplied to the coils, the movable body can be rotated such that the objective lens defines any desired angle with respect to the optical disc and that the tilt of the optical axis of the light beam with respect to the optical disc can be corrected. This movable body works as an objective lens driver and can not only correct the tilt but also drive the objective lens in the focus direction or in the tracking direction.
For example, Japanese Laid-Open Publication No. 4-366429 discloses an objective lens driver such as that shown in FIG. 1A. In FIG. 1A, the arrow F (i.e., the direction coming out of the paper) indicates the focus direction, the arrow T indicates the tracking direction, the arrow K indicates a tangential direction and the arrow R indicates a tilt direction. The focus, tracking and tangential directions F, T and K cross each other at right angles and respectively represent each coordinate axis of a three-dimensional orthogonal coordinate system. As shown in FIG. 1A, an objective lens 101 is secured to a lens holder 102. Four focus coils 103a, 103b, 103c and 103d are separately fixed to the lens holder 102 so as to be symmetrical to each other with respect to not only a plane that includes the optical axis J of the objective lens 101 and that is parallel to the tracking direction T but also a plane that includes the optical axis J of the objective lens 101 and that is perpendicular to the tracking direction T. Also, as shown in FIG. 1B, four tracking coils 108a, 108b, 108c and 108d are further provided on a plane of the lens holder 102, which is defined by the tracking and focus directions T and F. The lens holder 102 is supported to a base 107 by way of elastic supporting members 106 so as to freely shift not only in the tracking and focus directions T and F but also swing in the tilt direction R as well.
Furthermore, two magnets 105a and 105b are provided on the base 107 and outside of the focus coils 103a through 103d so as to be symmetrical with respect to the plane that includes the optical axis J of the objective lens 101 and that is parallel to the tracking direction T and so as to be spaced apart from the focus coils 103a through 103d. Yokes 104a, 104b, 104c and 104d, which form integral parts of the base 107, are provided inside of the focus coils 103a, 103b, 103c and 103d, respectively. The yokes 104a through 104d and magnets 105a and 105b together make up a magnetic circuit.
In an objective lens driver with such a configuration, if current is supplied through the focus coils 103a through 103d such that electromagnetic force with the same intensity and same direction is generated from each of the four focus coils 103a through 103d, the objective lens 101 and lens holder 102 can be shifted in the focus direction F without tilting the optical axis J of the objective lens 101 at all. As a result, a focus operation can be carried out just as intended.
On the other hand, if current is supplied through the tracking coils 108a and 108b such that electromagnetic force with the same intensity and same direction is generated from the tracking coils 108c and 108d, then force is generated in the tracking direction T among these coils. In this manner, the lens holder 102 can be shifted in the tracking direction T.
Furthermore, if the amount of current flowing between the focus coils 103a and 103c is set different from that of current flowing between the focus coils 103b and 103d, then the objective lens 101 can be rotated around an axis that is parallel to the tangential direction K. That is to say, the optical axis J can be tilted with respect to the data storage layer of the optical disc. As a result, a tilt correcting operation can be carried out based on a difference in electromagnetic force generated.
In the conventional objective lens driver, to allow the lens holder 102 to shift in the tracking and tilt directions T and R, a gap G needs to be provided between the yokes 104a through 104d and the focus coils 103a through 103d. Also, to achieve sufficient focus driving force, the length L1 of the focus coils 103a through 103d as measured in the tracking direction T needs to be at least equal to a predetermined value. Furthermore, the thickness L2 of the wound focus coils 103a through 103d also needs to be taken into consideration. In addition, to make the tilt correction, each pair of focus coils 103a and 103b or 103c and 103d needs to be arranged in the tracking direction T. For these reasons, the width L of the objective lens driver as measured in the tracking direction T should be at least equal to a predetermined value. For example, L≧13 mm needs to be satisfied. The width of the optical head as measured in the tracking direction is also at least equal to this value. That is to say, to make the tilt correction, the optical head should have a rather big width.
Recently, optical disc drives are built in various types of mobile electronic appliances. As a result, to make optical discs even more handy and lightweight and to increase the maximum amount of data that can be stored on a single optical disc, an optical disc with an increased storage density and with a smaller diameter is now in high demand. In an optical disc with such a small diameter, however, the innermost track thereof has a rather small radius. For example, in an optical disc that has an even smaller diameter than that of a mini disc (MD) (with a diameter of 64 mm), the innermost track thereof has a radius of 10 mm.
To process such a small optical disc, the optical disc drive needs to have the tilt correcting mechanism and shift the objective lens to any location on the innermost track with the radius of about 10 mm. In the optical disc drive, however, a spindle motor for rotating a given optical disc and a turntable to mount the optical disc thereon are provided around the center of the optical disc loaded. Accordingly, if the optical head has a big width in the tracking direction, then the optical head may contact or interfere with the turntable or spindle motor unintentionally.
More specifically, to shift the objective lens to the location defined by the radius of about 10 mm as described above, the optical head and the objective lens driver need to have a width La of about 10 mm or less as measured in the tracking direction T. Accordingly, if an optical disc drive including the conventional objective lens driver is used to read and/or write data from/on such an optical disc with a small diameter and high storage density, then the optical head or the objective lens driver may interfere with the turntable and other members, thus making it difficult to read or write data from/on the innermost and surrounding tracks of the optical disc.